The present invention relates to an illumination arrangement for providing polarized light to a flat panel display and a display comprising such an illumination arrangement.
Flat panel displays of the liquid crystal type (i.e. LCDs) are today widely used in products of very different character and fields of application. Many applications include small portable battery powered units such as portable telephones and computers. An obviously important feature of such applications is that of power consumption, a factor which may be decisive for a consumer when selecting between products of different manufacturers.
A requirement for a backlit LCD is that it has to be illuminated with linearly polarized light in order to function properly. In order to obtain polarized light, means for polarizing xe2x80x9cordinaryxe2x80x9d non-polarized light must be used. This usually entails putting so-called polarizers in the path of the non-polarized light having the effect of removing a large part of the incoming light. Since many of the applications involve portable equipment, battery power is required. However efficient the light sources may be, low power consumption will always be a key consideration.
Hence, a general problem in the field of the present invention is that there is a lack of efficiency in present day illumination arrangements for providing polarized light.
U.S. Pat. No. 5,845,035 discloses an illumination system for a flat panel picture display device. The system disclosed comprises a light wave-guide consisting of two layers, one layer which is optically anisotropic having indices of refraction n0 and ne, and a second layer which is optically isotropic having an index of refraction ni.
It can be shown that the polarization separation in the configuration of U.S. Pat. No. 5,845,035 only occurs under limited conditions and only for limited ranges of the angle of incidence xcex8 of the incoming light, the angle of incidence xcex8 being the angle between the normal direction of the layers and the incident light.
In order to obtain polarization separation, i.e. obtain polarized light for illumination of e.g. a LCD, U.S. Pat. No. 5,845,035 shows that the index of refraction ni of the second layer should match either of the indices of refraction n0, ne of the first layer.
These restrictions mean that, for an illumination system according to prior art to be efficient, i.e. in order to obtain efficient polarization separation at the interface, the light entering the system has to be collimated within a certain range, as shown in U.S. Pat. No. 5,808,709. This, needless to say, adds to the complexity of the illumination system or, if no collimation is applied, severe degradation of the efficiency of the system.
As will be illustrated in more detail below, it can be shown that, in order to get polarization separation at the interface between the layers there exists a restriction on the angle of incidence xcex8 in that it has to be larger than a critical angle of incidence xcex8c. This implies a restriction on the angle of incidence xcex8i at the incoupling facet of the illumination system as will be illustrated in more detail below.
An object of the present invention is thus to overcome the problems related to prior art systems as discussed above. To this end, an illumination arrangement and display system are provided as claimed below.
An illumination arrangement according to the present invention thus comprises an optically anisotropic substrate with a top surface and a side face. The side face receives incoming light (L), preferably from an external light source. Along the substrate is a top layer located. The top layer has a bottom surface that interfaces the top surface of the substrate, for example being in abutment with the surface of the substrate.
The substrate and the top layer have refractive indices such that a first polarization component of light directed into the substrate through said side face will be internally reflected back into the substrate at the interface between the substrate and the top layer, notwithstanding the incident angle at said side face.
A second polarization component of light directed into the substrate through the side face will be refracted into the top layer at the interface between the substrate and the top layer, notwithstanding the incident angle at said side face.
In some more detail, polarization of incoming light is obtained in that a first index of refraction of the substrate along a first direction relates to a first index of refraction of the top layer along the first direction according to the expression ns1xe2x89xa7sqrt(1+nt1*nt1). This has the effect that, for values of the first index of refraction of the substrate along the first direction being higher than sqrt(1+nt1*nt1), light experiencing these refractive indices and having any angle of incidence with respect to the side face of the substrate will be subject to total internal reflection in the interface between the substrate and the top layer and thus not reaching the top layer.
Furthermore, a weighted average value of a second index of refraction of the substrate along a second direction and a third index of refraction of the substrate along a third direction, is lower than a weighted average value of a second index of refraction of the top layer along the second direction and a third index of refraction of the top layer along the third direction. This has the effect of light experiencing these indices being refracted into the top layer, for all angles of incidence with respect to the side face of the substrate, i.e. regardless of the angle xcex8.
Embodiments of an illumination arrangement according to the present invention include combinations of optically anisotropic substrates with either isotropic or anisotropic top layers.
Another aspect of the present invention is that of a display system comprising an illumination arrangement as disclosed above and which also may comprise a light source arranged to illuminate the side face (212) of the substrate. A display, such as a flat panel LCD is arranged to receive polarized light emanating from the top layer. In such a case the LCD is a separate unit abutting the illuminating arrangement. However, it is possible according to yet another aspect of the invention, to incorporate a display unit, such as a LCD, within the top layer, in which case it receive polarized light from the interface between the substrate and the top layer.
An advantage with an illumination arrangement as disclosed is that it provides efficient polarization and it may by virtue of that fact be used in portable devices, such as computers, telephones etc. where power consumption is a critical factor.
Another advantage of such an arrangement is that it provides polarized light for a wide range of angles of incidence for incoming light which eliminates the need for collimating units such as lenses etc. between the source of light and the substrate of the arrangement. This means that display systems may be of low complexity containing fewer parts than systems according to prior art.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.